Synchronous satellites have been employed for a number of years as relay stations for communications between various points on the earth's surface. Their use facilitates communication at microwave frequencies where very broad-band operation and resulting large channel capacities are attainable, but which, on the other hand, require essentially line-of-sight transmission paths for acceptable reliability. Non-synchronous satellites have been and still are used as communications relay vehicles. However, synchronous satellites, which are positioned at fixed points above the Equator, are generally preferred because they are essentially stationary with respect to the earth's surface. Thus they need not be tracked by the ground station antennas, nor do they require compensation for doppler shifts, which are encountered with the non-synchronous satellites.
Moreover a single synchronous satellite can serve as a relay whereas, if non-synchronous satellites are used in the same application, a number of them are required because each one is continuously moving into or out of range of the ground stations. We shall therefore describe the present invention in terms of synchronous satellites, although it should be understood that the invention is also applicable to non-synchronous satellite systems.
In one presently operating satellite relay system the satellite carries several transponders, i.e. broad-band amplifiers and frequency translators, which amplify the signals received from earth stations and re-transmit the signals toward the earth for reception by other earth stations. Because of its broad-band capability each transponder can accommodate simultaneous transmissions in a relatively large number of communications channels having different carrier frequencies.
With this arrangement, each terminal station on the ground is constructed with the capability of transmitting and receiving in each of the channels. Each time a terminal station is to begin a transmission, an unused channel is assigned as a communications path between that station and the terminal station that is to receive the transmission. In one proposed system, channel assignment is accomplished by means of a ground control station that keeps track of the unused channels and assigns them on demand to stations that are about to commence message transmissions. The control station, which does not itself participate in the transmission of messages between the terminal stations, is "connected" to the terminal stations by a designated channel that serves as an "order wire."
A system presently in use, known as the SPADE system, differs from the foregoing one in that each of the terminal stations keeps track of all the unused channels and selects one itself when it is about to commence transmission. A control station is still used, but in this case its only function is to ascertain when two terminal stations have selected the same channel and, in such cases, cause the terminal stations to make new selections.
Two characteristics of these systems are of particular significance in connection with the present invention. The first of these, whose relevance will become apparent from the ensuing description, is that the maximum number of subscribers that can transmit at any given time exactly equals the number of available channels. The number of available channels is a limitation imposed by the ratio of (a) the total power that can be transmitted by the satellite transponders to (b) the required power per channel. The transponders have a bandwidth limitation and optimum design considerations dictate that this limitation is reached at the point that the maximum output power is reached. Thus the bandwidth corresponds exactly to the channel spacing multiplied by the number of channels.
In practice this imposes an absolute limit on the number of channels in a given system. Generally only a fraction of the terminals will have messages to transmit at any given time. Therefore, the total number of subscribers can be substantially greater than the number of available channels. However, on occasion all the channels will be in use simultaneously and the system will then be operating at full capacity. If a ground terminal then requests assignment of a channel for message transmission, the request must be denied. Either the number of subscribers must be severely limited to make this a very rare occurrence or the terminal stations must be provided with equipment for storing blocked messages during intervals in which there are no available channels.
The second characteristic is the relatively high cost of each of the terminal stations. This tends to limit the number of terminal stations, with a large number of subscribers being connected to each such station. As a result, a substantial part of the operating cost of the system involves lengthy ground links, either by wire or microwave, between the subscribers and the terminal stations. The principal object of the present invention is to reduce the cost of the terminal stations and increase their number so as to reduce the lengths of the ground links and thereby effect a material reduction in the overall expense.